Multimode optical fiber having improved bending losses

Abstract

The present invention embraces a multimode optical fiber that includes a central core having an alpha refractive index profile with respect to an outer optical cladding and a depressed trench positioned between the central core and the outer optical cladding. The central core's refractive index at its periphery is the same as the outer cladding's refractive index. Typically, an inner cladding is positioned between the central core and the depressed trench. The optical fiber achieves reduced bending losses without significantly increasing numerical aperture.

The invention claimed is: 
     
       1. A multimode optical fiber, comprising:
 a central core positioned within an outer optical cladding, said central core having a radius r 1  and an alpha refractive index profile with respect to said outer optical cladding; 
 an inner cladding positioned between said central core and said outer optical cladding, said inner cladding having a radius r 2  and a refractive index difference Δn 2  with said outer optical cladding; 
 a depressed trench positioned between said inner cladding and said outer optical cladding, said depressed trench having a width W t  and a refractive index difference Δn t  with said outer optical cladding; 
 wherein, at said central core's radius r 1 , the refractive index difference between said central core and said outer optical cladding is zero; 
 wherein said depressed trench's refractive index difference Δn t  is between about −7×10 −3  and −10×10 −3 ; 
 wherein said depressed trench's width W t  is between 3 microns and 4.5 microns; and 
 wherein said depressed trench has a volume V of between about −30 microns and −40 microns as defined by the expression V=1000×W t ×Δn t . 
 
     
     
       2. The optical fiber according to  claim 1 , wherein the numerical aperture of the optical fiber exceeds that of a comparative optical fiber without a depressed trench by no more than 0.010. 
     
     
       3. The optical fiber according to  claim 1 , wherein said depressed trench's refractive index difference Δn t  is between −7.5×10 −3  and −9.5×10 −3 . 
     
     
       4. The optical fiber according to  claim 1 , wherein said depressed trench has a rectangular refractive index profile. 
     
     
       5. The optical fiber according to  claim 1 , wherein the difference between said inner cladding's radius r 2  and said central core's radius r 1  is less than about 5 microns. 
     
     
       6. The optical fiber according to  claim 1 , wherein:
 said central core has a diameter of 62.5+/−3 microns; and 
 the optical fiber has a numerical aperture of 0.275+/−0.015. 
 
     
     
       7. The optical fiber according to  claim 1 , wherein:
 said central core has a diameter of 50+/−3 microns; and 
 the optical fiber has a numerical aperture of 0.2+/−0.015. 
 
     
     
       8. The optical fiber according to  claim 1 , wherein, for two turns around a radius of curvature of 15 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 40 percent less than those of a comparative optical fiber without a depressed trench. 
     
     
       9. The optical fiber according to  claim 1 , wherein, for two turns around a radius of curvature of 10 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 30 percent less than those of a comparative optical fiber without a depressed trench. 
     
     
       10. The optical fiber according to  claim 1 , wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 20 percent less than those of a comparative optical fiber without a depressed trench. 
     
     
       11. The optical fiber according to  claim 1 , wherein, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 20 percent less than those of a comparative optical fiber without a depressed trench. 
     
     
       12. A cable containing one or more optical fibers according to  claim 1 . 
     
     
       13. A multimode optical fiber, comprising:
 a central core positioned within an outer optical cladding, said central core having a radius r 1  and an alpha refractive index profile with respect to said outer optical cladding; 
 an inner cladding immediately surrounding said central core and positioned within said outer optical cladding, said inner cladding having a radius r 2  and a refractive index difference Δn 2  with said outer optical cladding; 
 a depressed trench positioned between said inner cladding and said outer optical cladding, said depressed trench having a width W t  of between 2.0 microns and 4.5 microns and a refractive index difference Δn t  with said outer optical cladding that is between about −6×10 −3  and −15×10 −3 ; 
 wherein, at said central core's radius r 1 , the refractive index difference between said central core and said outer optical cladding is zero; 
 wherein said depressed trench has a volume V of between about −30 microns and −40 microns as defined by the expression V=1000×W t ×Δn t ; and 
 wherein the optical fiber has a numerical aperture that exceeds the numerical aperture of a comparative optical fiber without a depressed trench by no more than 0.010. 
 
     
     
       14. The optical fiber according to  claim 13 , wherein, for two turns around a radius of curvature of 15 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 40 percent less than those of a comparative optical fiber without a depressed trench. 
     
     
       15. The optical fiber according to  claim 13 , wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 20 percent less than those of a comparative optical fiber without a depressed trench. 
     
     
       16. A multimode optical fiber, comprising:
 a central core positioned within an outer optical cladding, said central core having a radius r 1  and an alpha refractive index profile with respect to said outer optical cladding; 
 a depressed trench immediately surrounding said central core and positioned within said outer optical cladding, said depressed trench having a width W t  and a refractive index difference Δn t  with said outer optical cladding that is greater than −9×10 −3 ; 
 wherein, at said central core's radius r 1 , the refractive index difference between said central core and said outer optical cladding is zero; 
 wherein said depressed trench's width W t  is between 3 microns and 4.5 microns; 
 wherein said depressed trench has a volume V of between about −30 microns and −40 microns as defined by the expression V=1000×W t ×Δn t ; and 
 wherein the optical fiber has a numerical aperture that exceeds the numerical aperture of a comparative optical fiber without a depressed trench by no more than 0.010. 
 
     
     
       17. The optical fiber according to  claim 1 , wherein the difference between said inner cladding's radius r 2  and said central core's radius r 1  is between about 2 microns and 4 microns. 
     
     
       18. The optical fiber according to  claim 1 , wherein said inner cladding immediately surrounds said central core, and said depressed trench immediately surrounds said inner cladding. 
     
     
       19. The optical fiber according to  claim 13 , wherein said depressed trench's refractive index difference Δn t  is between about −7×10 −3  and −10×10 −3 . 
     
     
       20. The optical fiber according to  claim 13 , wherein said depressed trench's refractive index difference Δn t  is between about −8×10 −3  and −12×10 −3 . 
     
     
       21. The optical fiber according to  claim 13 , wherein the optical fiber according to  claim 13 , wherein said depressed trench has a rectangular refractive index profile. 
     
     
       22. The optical fiber according to  claim 13 , wherein the difference between said inner cladding's radius r 2  and said central core's radius r 1  is between about 2 microns and 5 microns. 
     
     
       23. The optical fiber according to  claim 13 , wherein, for two turns around a radius of curvature of 10 millimeters at a wavelength of 850 nanometers, the optical fiber's bending losses are at least 35 percent less than those of a comparative optical fiber without a depressed trench.